The United States-Mexico border fence may block more wildlife border crossings than people crossings. The already limited populations of pygmy owls and bighorn sheep will likely be among those further threatened by the fence, says a new study.

"In some respects, it's as obvious as the nose on your face: If you put up a 20-foot chunk of steel, things probably aren't going to move across it, but we have to show it," said Paul Beier of Northern Arizona University in Flagstaff, who was not a part of the study. "It's kind of a shame that we need to prove the obvious, but we do. So this is an important paper in that regard."

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Since the U.S. government authorized construction of a fence along 700 miles of the border between the U.S. and Mexico in 2006, scientists have raised concerns that a person-tight fence would also exclude many species that rely on habitat on either side of the border. The new research suggests such concerns are valid.